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Block 2 CTB > The heart as an electrical pump > Flashcards

The heart as an electrical pump Flashcards

1
Q

What are the 6 main components of the cardiac conduction system?

A

Sinoatrial node (SAN)

Internodal tracts

Atrioventricular node (AVN)

Bundle of His

Bundle branches (left & right)

Purkinje fibres

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2
Q

Describe the location of the SAN.

A

Posterior and upper wall of the right atrium, close to the opening of the SVC

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3
Q

The SAN has intrinsic automaticity – what does this mean?

A

It can spontaneously generate action potentials

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4
Q

What is the frequency of spontaneous action potentials generated by the SAN?

A

Once every second

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5
Q

Why can’t electrical signal propagate from the atrium to the ventricles directly?

A

Because of the fibrous cardiac skeleton – it electrically isolates the atria from the ventricles

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6
Q

Why is it important that electrical signal is transmitted via the AVN as opposed to directly from the atria to the ventricles?

A

The AV node introduces a physiological delay of approximately 100ms in conduction, allowing time for the atria to contract and complete ventricular filling before the ventricles contract

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7
Q

List the conduction velocities of the atria, AV node, His-Purkinje, ventricle

A

Atria – 1 m/s

AV node – 0.01 – 0.05 m/s

His-Purkinje – 2-4 m/s

Ventricle – 1 m/s

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8
Q

List the 3 latent pacemakers of the heart and their discharge rates.

A

AV node – 40-50 bpm

Bundle of His – 40-50 bpm

Purkinje fibres – 20 bpm

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9
Q

Ectopic pacemakers can arise from what?

A

Hyperexcitability of atrial and ventricular myocytes

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10
Q

Describe the pattern of contraction in the ventricles.

A

Contraction happens from the apex to the base

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11
Q

What is the resting membrane potential of myocytes?

A

-65 to -90 millivolts (mV)

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12
Q

What are the 2 types of cardiac action potentials?

A

Slow response potentials
Fast response potentials

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13
Q

Where are slow response potentials found?

A

SAN and AVN

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14
Q

Describe the speed of depolarization in slow response potentials and what this is due to.

A

Slowly depolarizing cells due to unstable resting membrane potential (‘pacemaker’ potential)

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15
Q

Describe the speed of depolarization in slow response potentials and what this is due to.

A

Rapidly depolarizing cells due to stable resting membrane potential

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16
Q

Cardiac action potentials have how many phases?

A

5 phases – Phase 0 – Phase 4

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17
Q

What is Phase 0 of the cardiac action potential?

A

Depolarisation

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18
Q

Cardiac action potentials are divided into phases according to what?

A

Ion movements

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19
Q

What is Phase 1 of the cardiac action potential?

A

Rapid repolarization

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20
Q

What is Phase 2 of the cardiac action potential?

A

Plateau

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21
Q

What is Phase 3 of the cardiac action potential?

A

Repolarization

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22
Q

What is Phase 4 of the cardiac action potential?

A

Resting

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23
Q

What phases do slow response cardiac action potentials have?

A

Phases 0, 3, 4

24
Q

What phases do fast response cardiac action potentials have?

A

Phases 0, 1, 2, 3, 4

25
Q

Describe what happens during each phase of slow response action potentials.

A

Phase 0 – Ca2+ influx causes slow depolarization

Phase 3 – K+ efflux causes repolarization

Phase 4 – membrane is permeable to Na+ and K+. Net inward current causes slow depolarization

26
Q

Which current gives the pacemaker cells their intrinsic automaticity?

A

Funny current

27
Q

In terms of the parasympathetic nervous system – which nerve innervates the heart, which specific areas does it innervate, which neurotransmitter, which receptors, and what effect does it have?

A

Vagus nerve – mostly innervates SAN and AVN

Acetylcholine binds to muscarinic M2-cholinergic receptors, slowing the heart rate

28
Q

In terms of the sympathetic nervous system – which specific area of the heart does it innervate, which mediator, which receptors, and what effect does it have?

A

Innervates conduction system and myocardium

Noradrenaline binds to B1-adrenoreceptors, increasing the heart rate

29
Q

Describe the 3 ways in which the parasympathetic nervous system slows heart rate.

A

ACh reduces the influx of calcium ions, which leads to a less steep Phase 4, meaning more time is required to reach the threshold

ACh causes K+ channels to open, increasing K+ efflux which leads to hyperpolarization, so more time is required to reach the threshold

Calcium channels are modulated – the threshold for activation is increased.

30
Q

Define negative chronotropy and where it occurs.

A

A decrease in heart rate – a slowing of the frequency at which the heart beats

Occurs in the SAN

31
Q

Define negative dromotropy and where it occurs.

A

A decrease in the speed of electrical conduction through the heart

Occurs in the AVN

32
Q

What is the overall effect of the parasympathetic nervous system on the SAN?

A

Decreases pacemaker firing

33
Q

What is the overall effect of the parasympathetic nervous system on the AVN?

A

Decreases conduction velocity

34
Q

Describe the 2 ways in which the sympathetic nervous system increases heart rate.

A

Noradrenaline increases the influx of calcium, which leads to a steeper Phase 4, causing faster depolarization

Calcium channels are modulated – the threshold for activation is decreased

35
Q

What is the overall effect of the sympathetic nervous system on the SAN?

A

Increases pacemaker firing

36
Q

What is the overall effect of the sympathetic nervous system on the AVN?

A

Increases conduction velocity

37
Q

Where are fast response potentials found?

A

Atrial + ventricular myocytes

Purkinje fibres

38
Q

Describe what happens during each phase of fast response action potentials.

A

Phase 0 – Rapid influx of Na+ causes rapid depolarization

Phase 1 – initial K+ efflux, causing rapid and transient repolarization

Phase 2 – influx of Ca2+ balances K+ efflux, creating a sustained plateau

Phase 3 – K+ efflux causes repolarization

Phase 4 – stable resting membrane potential

39
Q

What are the 2 refractory periods in a fast response potential?

A

Absolute refractory period

Relative refractory period

40
Q

What cannot happen during the absolute refractory period?

A

No new action potential can be generated

41
Q

When does the absolute refractory period occur?

A

From the beginning of Phase 0 through to Phase 3

42
Q

When does the relative refractory period occur?

A

From Phase 3 through to the early part of Phase 4

43
Q

Describe the generation of action potentials in the relative refractory period.

A

Action potentials can be generated but a stronger than normal stimulus is needed

44
Q

Give 2 reasons why the refractory period in fast response potentials is important.

A

Ensures unidirectional propagation of the action potential

Ensures adequate time for ventricular filling prior to subsequent contraction

45
Q

What is the outer membrane of cardiomyocytes called?

A

Sarcolemma

46
Q

What are the thick filaments of sarcomeres made of?

A

Myosin

47
Q

What are the thin filaments of sarcomeres made of?

A

Actin, tropomyosin, and troponin

48
Q

In cardiomyocytes, what are transverse tubules and what is their function?

A

Invaginations of the sarcolemma that extend deep into the cardiomyocyte

They allow the depolarization of the membrane to penetrate muscle fibre

49
Q

What is the sarcoplasmic reticulum?

A

Intracellular organelle which stores calcium

50
Q

Cardiomyocytes connect to each other via what?

A

Intercalated discs

51
Q

Which 2 specialized structures are present in cardiomyocytes which provide mechanical support and electrical coupling?

A

Desmosomes and gap junctions

52
Q

Which proteins are gap junctions composed of?

A

Connexins

53
Q

What is excitation-contraction coupling?

A

The mechanism that translates a cardiac action potential into muscle contraction

54
Q

Describe what happens during excitation-contraction coupling.

A

T tubule membrane depolarisation. This causes the opening of voltage-gated calcium channels. A small influx of calcium binds to and activates ryanodine receptors on the sarcoplasmic reticulum, causing a large release of calcium. Calcium binds to troponin C, causing contraction. Calcium removal from the cytoplasm leads to relaxation – it is actively pumped back into the SR by SR Ca2+-ATPase (SERCA) and removed from the cell via NCX

55
Q

What is the process called where calcium binding to ryanodine receptors on the SR releases calcium?

A

Calcium-induced calcium release

Block 2 CTB (16 decks)

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